Atomistic Simulations of Poly(ethylene oxide) in Water and an Ionic Liquid at Room Temperature

Osmolyte-induced conformational stabilization of a hydrophobic polymer

Elucidating the mechanistic role of osmolytes on conformations of hydrophobic prototypical macromolecules in principle is the stepping stone towards understanding the effect of osmolytes on proteins. Motivated by this, we use equilibrium simulations and umbrella sampling techniques to dissect the underlying mechanism of osmolyte-induced conformational stability of a hydrophobic polymer. Our results unveil a remarkable osmolyte-dependent conformational stabilization of the polymer. In an aqueous solution of 4 M choline chloride (ChCl), the polymer has an even more compact structure than in water. On the other hand, an aqueous solution of 8 M urea stabilizes the extended state of the polymer. Interestingly, the polymer adopts an intermediate hairpin conformation in a mixed osmolyte solution of 4 M ChCl and 8 M urea in water due to the interplay of ChCl and urea. Our simulations identify the relative accumulation of water and the hydrophilic part of choline or preferential binding of urea near the collapsed and the extended states, respectively. Analyses split out the enthalpic and entropic contributions to the overall free energy. This decides the stabilization of the preferred conformation in the chosen osmolyte solution. Our simulations show that in an aqueous solution of ChCl, the hairpin state is stabilized by entropy gain. In contrast, the enthalpic contribution stabilizes the hairpin state in mixed environments. However, a collapsed state is energetically not favored in the presence of urea. In brief, via employing an in silico approach, the current findings indicate the importance of osmolytes in stabilizing the conformational states of hydrophobic polymers.

Impact of a poly(ethylene glycol) corona block on drug encapsulation during polymerization induced self-assembly

Polymerization induced self-assembly (PISA) provides a facile platform for encapsulating therapeutics within block copolymer nanoparticles. Performing PISA in the presence of a hydrophobic drug alters both the nanoparticle shape and encapsulation efficiency. While previous studies primarily examined the interactions between the drug and hydrophobic core block, this work explores the impact of the hydrophilic corona block on encapsulation. Poly(ethylene glycol) (PEG) and poly(2-hydroxypropyl methacrylate) (PHPMA) are used as the model corona and core blocks, respectively, and phenylacetic acid (PA) is employed as the model drug. Attachment of a dithiobenzoate end group to the PEG homopolymer - transforming it into a macroscopic reversible addition-fragmentation chain transfer agent - causes the polymer to form a small number of nanoscopic aggregates in solution. Adding PA to the PEG solution encourages further aggregation and macroscopic phase separation. During the PISA of PEG-PHPMA block copolymers, inclusion of PA in the reaction mixture promotes faster nucleation of spherical micelles. Although increasing the targeted PA loading from 0 to 20 mg mL-1 does not affect the micelle size or shape, it alters the drug spatial distribution within the PISA microenvironment. PA partitions into either PEG-PHPMA micelles, deuterium oxide, or other polymeric species - including PEG aggregates and unimer chains. Increasing the targeted PA loading changes the fraction of drug within each encapsulation site. This work indicates that the corona block plays a critical role in dictating drug encapsulation during PISA.

Structural, energetic and dynamic investigation of poly(ethylene oxide) in imidazolium-based ionic liquids with different cationic structures

In this work, two imidazolium-based ionic liquids (ILs) with different cations including dications (DIL) and monocations (MIL) were blended with poly(ethylene oxide) (PEO). The influence of ILs' structure on the structural and dynamic properties of a PEO/IL system was investigated by molecular dynamics (MD) simulation and density functional theory (DFT) methods. The simulation results show that DIL exhibits weaker interaction with PEO than MIL due to a stronger IL aggregation effect. The intermolecular interaction also makes the PEO chain tend to organize around the imidazolium ring of ILs, which causes the conformational entropy loss. Compared with PEO/MIL, this phenomenon is more significant in PEO/DIL because of the double positive centers of the dication and a longer hydrogen bond lifetime. MD simulation also demonstrates that DIL could act as a "crosslinker" to promote the formation of a physical crosslinking network which has strong dependence on the concentration of IL. The competition between physical crosslinking and plasticizing effects induces non-monotonic variations of relaxation time in PEO/DIL, which is consistent with its unusual change of the glass transition temperature (Tg). Despite stronger hydrogen bonding interactions between PEO and MIL demonstrated by atom-in-molecules (AIM) and reduced density gradient (RDG) analysis, the segmental mobility is slower in PEO/DIL according to the MSD curve. These differences in multiple structural or energetic factors finally lead to different conductive mechanisms and hence obtain different ionic conductivities.

Conformations and Solvation of Synthetic Polymers in Water by Generalized Born Implicit-Solvent Molecular Dynamics Simulations: Stereoisomers of Poly(acrylic acid) and Poly(methacrylic acid)

We present the GB-OBC model as an approach for implicit-solvent MD simulations of a synthetic macromolecule in water. The model is tested and found to be successful in reproducing the chain dimensions and predicting the free energy of solvation of carboxylic acid vinyl polymers. The influence of stereochemistry and the hydrophobic nature of the polymer was investigated as a function of chain length (20 < N < 600) for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA). The dimensionless parameters of the GB-OBC model were parameterized to be applicable to PAA and PMA. Scaling relations for chain dimensions obtained using implicit-solvent MD simulations in this study are in good agreement with those from experiments, theory of solvated chains in good solvents, and all-atom MD simulations in explicit water. Results show that ⟨R_g²⟩/NL² is greater for the atactic chain as compared to the isotactic chain, for PAA as well as PMA. ⟨R_g²⟩/NL² values of chains attain constancy in water for N = 200, with the values being greater for PMA. The PMA chain is conformationally more perturbed than the PAA chain, for both isotactic and atactic stereochemistry. The solvation free energy ΔG_hyd of PAA and PMA in water is negative for all chain lengths (N = 20-600) and becomes more favorable with an increase in molecular weight. The ΔG_hyd values for isotactic and atactic chains are identical at lower values of N but differ slightly for N > 300. Irrespective of the hydrophobic nature of the polymer, the atactic chain is thermodynamically more soluble in water as compared to the isotactic chain. The isotactic chain is less hydrophilic as compared to the atactic chain due to the closer proximity of the COOH groups along the backbone. This implicit solvent method is an effective way to accurately simulate the configurational properties and solvation of synthetic polymers in water.

Influence of ionic liquids on the chain dynamics and enthalpy relaxation of poly(methyl methacrylate)

Imidazolium ionic liquids (ILs) with various alkyl chain lengths on the cations ([C_nmim]⁺, n = 2, 4 and 8) and different combined anions ([TFSI]^- and [PF₆]^-) were blended with poly(methyl methacrylate) (PMMA), and the effects of the IL structure on the chain dynamics of PMMA were experimentally investigated by rheology and DSC measurements combined with a simulation method. The results indicate that the interaction between PMMA and ILs becomes stronger as the alkyl chain length on the imidazolium ring increases or the anion changes from [PF₆]^- to [TFSI]^-. As a result, a higher critical entanglement concentration and a larger entanglement molecular weight of PMMA were found in [C₈mim][TFSI] due to the stiffer conformation. Molecular dynamics (MD) simulations further demonstrated stronger interactions between PMMA and ILs with longer cationic alkyl chain lengths or [TFSI]^- anions, which showed smaller Flory-Huggins interaction parameters and larger radii of gyration, R_g. However, the larger size of alkyl chains or [TFSI]^- anions produced a larger free volume in the system as evidenced by positron annihilation lifetime spectroscopy (PALS), which competed with the molecular interaction and dominated the segmental motion. Therefore, a lower T_g and accelerated segmental relaxation were observed. Compared to alkyl chain length, the effect of anions on the interactions between ILs and PMMA is more prominent.

Structure and Intermolecular Interactions in Aqueous Solutions of Polyethylene Glycol

Aqueous solutions of polyethylene glycol are studied by small-angle neutron scattering over a broad range of polymer molecular masses and concentrations. The scattering data were modeled by a Gaussian chain form factor combined with random phase approximation, which provided good fits over the whole studied concentration range. The results showed that polyethylene glycol in the molecular mass range 0.4–20 kDa in water at physiological temperature T = 37 °C behaves like a random coil in nearly theta solvent conditions. The obtained results serve as a reference for the description of complex mixtures with PEG used in various applications.

Solutions and Condensed Phases of PEG2000 from All-Atom Molecular Dynamics

Extensive all-atom molecular dynamics studies of polyethylene glycol (PEG₂₀₀₀) when solvated and in the polymer bulk condensed phases were performed across a wide temperature range. We proposed two modified all-atom force field and observed the fate of the PEG₂₀₀₀ macromolecule when solvated in water, water with 4% ethanol, and ethyl acetate. In aqueous solutions, the macromolecule collapsed into a prolate spheroidal ball-like structure while adopting a rather elongated coiled structure in ethyl acetate. Inspection of the polymer-condensed phases across the 150-340 K temperature range enabled the atomistic view of the solid glass below the glass transition temperature of 230 K < T_g < 250 K and the rubber behavior above T_g. Predicted properties include the enthalpy, density, and cohesive energy temperature behavior, the specific heat, thermal expansivity, thermal compressibility, bulk modulus, and Hildebrand solubility parameter both below and above T_g. Within the polymer matrix, the PEG₂₀₀₀ macromolecules were entangled displaying a wide distribution of sizes that persisted when transitioning from the glass to the rubbery phases. Calculated properties agree very well with experiments when available or stand as crucial predictions while awaiting experimental measurement. Understanding the thermodynamics and structure of this useful polymer enables the efficient prediction of its behavior when building novel composite materials for nanomedicine and nanotherapeutics.

Phase Behavior of Poly(ethylene oxide) in Room Temperature Ionic Liquids: A Molecular Simulation and Deep Neural Network Study

The phase behavior of polymers in room temperature ionic liquids is a topic of considerable interest. In this work we study the phase diagram of poly(ethylene oxide) in four imidazolium ionic liquids (ILs) using molecular simulation. We develop united atom models for 1-butyl-2,3-dimethylimidazolium ([BMMIM]), 1-ethyl-2,3-dimethylimidazolium ([EMMIM]), and 1-ethyl-3-methylimidazolium ([EMIM]) in an analogous fashion to previously developed models for 1-butyl-3-methylimidazolium ([BMIM]) and tetrafluoroborate ([BF₄]) using symmetry-adapted perturbation theory. At high temperatures we obtain the coexistence concentrations using an interface method where the polymer and IL are simulated in a large elongated box, and an interface between coexisting phases is formed. At lower temperatures we use a deep neural network (DNN) method. The input descriptors for the DNN are the cohesive energy of mixing, the volume change of mixing, and the coordination numbers between cation and polymer, all of which are obtained from simulations of mixed systems at a series of temperatures. The DNN is trained by using the phase-separated systems at high temperatures and a mixed phase at low temperatures. The method predicts a lower critical solution temperature which decreases as the alkyl chain length on the cation is decreased, consistent with experiment. The simulations show that methylation of the cation has little effect on the phase diagram. This is in contrast to what is seen in experiments but could be because the polymer chains in the simulations are too short. At low temperatures the chains display two conformational motifs, namely a crown ether conformation and a ring conformation, each of which can wrap the chain around a single cation. This provides the entropic penalty for mixing and a reason for demixing as the temperature is raised. Such conformations might not be possible for longer chains. The combination of data-driven techniques and molecular simulation shows promise in the study of the phase behavior and physical properties of complex fluids.

Solvation and diffusion of poly(vinyl alcohol) chains in a hydrated inorganic ionic liquid

While the behavior of polyelectrolyte chains in aqueous salt solutions has been extensively studied, little is known about polar polymer chains in solvents with extremely high concentrations of inorganic ions, such as those found in ionic liquids (ILs). Here, we report on expansion, solvation and diffusion of poly(vinyl alcohol), PVA, chains in dilute solutions of a hydrated inorganic IL phase change material (PCM), lithium nitrate trihydrate (LNH). This solvent has an extremely high concentration of inorganic ions (≈18 M) with a low concentration of water molecules largely forming solvation shells of Li⁺ and NO₃^- ions, as shown using ATR-FTIR spectroscopy. Diffusion and hydrodynamic size of PVA chains of different molecular weights in this unusual solvent were studied using fluorescence correlation spectroscopy (FCS). A higher scaling exponent obtained from the molecular weight dependences of the diffusion coefficients of PVA chains as well as a lower overlap concentration (c*) of PVA in LNH solutions as measured by FCS suggest an expansion of the polymer coils in this solvent. We argue that enhanced solubility of PVA in LNH solutions is likely a result of increased rigidification of polymer chains due to the binding of solvated Li⁺ ions, which is demonstrated using ⁷Li NMR spectroscopy. We believe that an understanding of solvation and ion-binding capability can offer crucial insight into designing polymer-based shape stabilization matrices for inorganic PCMs.

Rheology of Concentrated Polymer/Ionic Liquid Solutions: An Anomalous Plasticizing Effect and a Universality in Nonlinear Shear Rheology

An anomalous plasticizing effect was observed in polymer/ionic liquid (IL) solutions by applying broad range of rheological techniques. Poly(ethylene oxide)(PEO)/IL solutions exhibit stronger dynamic temperature dependence than pure PEO, which is in conflict with the knowledge that lower-Tg solvent increases the fractional free volume. For poly(methy methacrylate)(PMMA)/IL solutions, the subtle anomaly was detected from the fact that the effective glass transition temperature Tg,eff of PMMA in IL is higher than the prediction of the self-concentration model, while in conventional polymer solutions, Tg,eff follows the original Fox equation. Observations in both solutions reveal retarded segmental dynamics, consistent with a recent simulation result (Macromolecules, 2018, 51, 5336) that polymer chains wrap the IL cations by hydrogen bonding interactions and the segmental unwrapping delays their relaxation. Start-up shear and nonlinear stress relaxation tests of polymer/IL solutions follow a universal nonlinear rheological behavior as polymer melts and solutions, indicating that the segment-cation interaction is not strong enough to influence the nonlinear chain orientation and stretch. The present work may arouse the further theoretical, experimental, and simulation interests in interpreting the effect of complex polymer-IL interaction on the dynamics of polymer/IL solutions.

Influence of molecular weight on ion-transport properties of polymeric ionic liquids

We report the results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-vinylimidazolium-hexafluorophosphate ionic liquids, studying the influence of the polymer molecular weight on the ion mobilities and the mechanisms underlying ion transport, including ion-association dynamics, ion hopping, and ion-polymer coordinations. With an increase in polymer molecular weight, the diffusivity of the hexafluorophosphate (PF₆^-) counterion decreases and plateaus above seven repeat units. The diffusivity is seen to correlate well with the ion-association structural relaxation time for pure ionic liquids, but becomes more correlated with ion-association lifetimes for larger molecular weight polymers. By analyzing the diffusivity of ions based on coordination structure, we unearth a transport mechanism in which the PF₆^- moves by "climbing the ladder" while associated with four polymeric cations from two different polymers.

Unusual phase separation and rheological behavior of poly(ethylene oxide)/ionic liquid mixtures with specific interactions

The phase separation behavior of poly(ethylene oxide) (PEO) in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was investigated by rheological, optical microscopy, FT-IR and DSC measurements. It is demonstrated that specific interactions, particularly the hydrogen bonding between PEO and the ionic liquids as evidenced by FT-IR, in which a subtle but apparent absorption peak shift near the phase transition appears, account for the unusual low critical solution temperature (LCST) phase separation. Unlike the typical trend in which the storage modulus G' simply increases with temperature near the phase boundary for polymer blends without specific interaction, in our study, a novel "V-shaped" rheological response is observed, namely a dip in G' followed by an upturn, especially at low PEO concentration (<50 wt%). The magnitude of the "V" dip has heating rate and frequency dependences, while Tr (the phase transition temperature) is almost unchanged with heating rate and frequency. Upon increasing the alkyl chain length on the imidazolium ring from an ethyl to a butyl, the "V-shape" becomes more prominent and shifts to higher temperature, which is consistent with the results of FT-IR and DSC, evidently due to the stronger hydrogen bonding interaction between PEO and [BMIM][BF4] than [EMIM][BF4]. This unusual "V" dip might be tentatively ascribed to the coupling effects of the breaking of the "hydrogen bonding cage" formed between PEO chains and IL molecules and dissolution of the heterogeneous clusters as verified by FT-IR and TEM, respectively, and the following upturn is dominated by the interface formation upon phase separation.

Polymer-ionic liquid ternary systems for Li-battery electrolytes: Molecular dynamics studies of LiTFSI in a EMIm-TFSI and PEO blend

This paper presents atomistic molecular dynamics simulation studies of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) in a blend of 1-ethyl-3-methylimidazolium (EMIm)-TFSI and poly(ethylene oxide) (PEO), which is a promising electrolyte material for Li- and Li-ion batteries. Simulations of 100 ns were performed for temperatures between 303 K and 423 K, for a Li:ether oxygen ratio of 1:16, and for PEO chains with 26 EO repeating units. Li(+) coordination and transportation were studied in the ternary electrolyte system, i.e., PEO16LiTFSI⋅1.0 EMImTFSI, by applying three different force field models and are here compared to relevant simulation and experimental data. The force fields generated significantly different results, where a scaled charge model displayed the most reasonable comparisons with previous work and overall consistency. It is generally seen that the Li cations are primarily coordinated to polymer chains and less coupled to TFSI anion. The addition of EMImTFSI in the electrolyte system enhances Li diffusion, associated to the enhanced TFSI dynamics observed when increasing the overall TFSI anion concentration in the polymer matrix.

Structural effect of glyme–Li+ salt solvate ionic liquids on the conformation of poly(ethylene oxide)

Conformation of poly(ethylene oxide) in solvate ionic liquids is affected by the solvent structure.

Dissolved chloride markedly changes the nanostructure of the protic ionic liquids propylammonium and ethanolammonium nitrate

The bulk nanostructure of 15 mol% propylammonium chloride (PACl) dissolved in propylammonium nitrate (PAN) and 15 mol% ethanolammonium chloride (EtACl) in ethanolammonium nitrate (EtAN) has been determined using neutron diffraction with empirical potential structure refinement fits.

Entropic Mechanism for the Lower Critical Solution Temperature of Poly(ethylene oxide) in a Room Temperature Ionic Liquid

Polymers exhibit interesting phase behavior in room temperature ionic liquids. For example poly(ethylene oxide) (PEO) displays a lower critical solution temperature (LCST) in [BMIM][BF₄] with a critical temperature and concentration that are only weakly dependent on molecular weight, contrary to the behavior of polymers in other solvents. To shed light on the mechanism of the LCST, we study the phase behavior of PEO in [BMIM][BF₄] using molecular dynamics (MD) simulations. The simulations show the signature of a phase transition as the temperature is increased. At low temperatures, interactions similar to a hydrogen bond are found between the imidazolium hydrogen and the PEO oxygen (HI-O H-bond) and the imidazolium hydrogen and the anion fluorines (HI-F H-bond). These interactions stabilize the mixed phase. A potential of mean force (PMF) analysis shows an entropic cost associated with the HI-O H-bond, which makes the bond formation unfavorable at higher temperatures, while the HI-F H-bond does not show a significant temperature dependence: This suggests that LCST phase separation is driven by the entropic penalty of the polymer for a PEO-cation hydrogen bond. We test the effect of scaling the charges on the [BMIM][BF₄]. Interestingly, the scaled charge force-field does not predict a phase separation at any temperature, thus, emphasizing the pitfalls of charge scaling for mixtures.

Investigation of Ion Channel Activities of Gramicidin A in the Presence of Ionic Liquids Using Model Cell Membranes

Ionic liquids (ILs) are considered to be green solvents because of their non-volatility. Although ILs are relatively safe in the atmospheric environment, they may be toxic in other environments. Our previous research showed that the cytotoxicity of ILs to biological organisms is attributable to interference with cell membranes by IL insertion. However, the effects of ILs on ion channels, which play important roles in cell homeostasis, have not been comprehensively studied to date. In this work, we studied the interactions between ILs and lipid bilayer membranes with gramicidin A ion channels. We used two methods, namely electrical and fluorescence measurements of ions that permeate the membrane. The lifetimes of channels were increased by all the ILs tested in this work via stabilizing the compressed structure of the lipid bilayer and the rate of ion flux through gA channels was decreased by changing the membrane surface charge. The former effect, which increased the rate of ion flux, was dominant at high salt concentrations, whereas the latter, which decreased the rate of ion flux, was dominant at low salt concentrations. The effects of ILs increased with increasing concentration and alkyl chain length. The experimental results were further studied using molecular dynamics simulations.

The binding and insertion of imidazolium-based ionic surfactants into lipid bilayers: the effects of the surfactant size and salt concentration

Imidazolium-based ionic surfactants with hydrocarbon tails of different sizes were simulated with lipid bilayers at different salt concentrations.

Conformation of poly(ethylene oxide) dissolved in the solvate ionic liquid [Li(G4)]TFSI

PEO dissolves in [Li(G4)]TFSI via coordination with Li⁺.